Vacuum-assisted venous drainage system with rigid housing and flexible reservoir

ABSTRACT

A vacuum-assisted venous drainage reservoir for cardiopulmonary bypass surgery with both hard and soft shell reservoirs. The system utilizes a wall vacuum or other source of negative pressure to create a negative pressure via a regulator within a sealed hard shell reservoir, or within a sealed housing surrounding a soft shell reservoir. The addition of a negative pressure in the venous return line enables the use of smaller cannulas suitable for minimally invasive surgery. The reservoir need not be positioned well below the patient as in conventional gravity venous drainage configurations, thus adding flexibility to the operating room layout and enabling a reduction in the extracorporeal blood prime volume needed. In one embodiment, a flexible membrane in the hard shell reservoir expands to contact the blood surface and reduce blood/air interactions. In another embodiment, a moisture trap is provided between the source of vacuum and hard shell reservoir to reduce environmental contamination. A volume sensor for the hard shell reservoir may be used in a feedback loop for controlling the vacuum, circulation pump, or other device. A pressure relief valve may be included in both systems for safety, and a vacuum stabilizer reduces the severity of large changes in vacuum pressure. A piece of air-permeable material may form a portion of the soft shell reservoir to vent air from within.

RELATED CASES

The is a divisional application Ser. No. 08/938,058 filed Sep. 26, 1997,which is now U.S. Pat. No. 6,017,493.

FIELD OF THE INVENTION

The present invention relates generally to reduced prime volumecardiopulmonary bypass systems and, more particularly, tovacuum-assisted venous drainage systems and methods.

BACKGROUND OF THE INVENTION

Cardiopulmonary bypass (CPB) surgery requires a perfusion system, orheart-lung machine, to maintain an adequate supply of oxygen in thepatient's blood during the surgery. An example of such a perfusionsystem is shown in FIG. 1. A venous return cannula is inserted in one ofthe veins leading directly to the heart and receives the “used” bloodfor rejuvenation through the perfusion system. The blood flows along aconduit (typically a transparent flexible tube) to a venous reservoirwhich may be combined with a cardiotomy reservoir. Commonly, a suckerextracts excess fluid from the chest cavity during the operation anddiverts the fluid, which may contain bone chips or other particulates,into the top of the cardiotomy reservoir. The cardiotomy sucker pullspooled blood from the chest cavity using a vacuum which may be generatedby a roller pump, for example. In addition, a vent cannula may bepositioned in the heart for suctioning other fluids during theoperation, those fluids also being directed to the cardiotomy reservoirthrough a roller pump. The fluid entering the cardiotomy reservoir isfirst filtered before being combined with the venous blood.

In contrast to the suction within the cardiotomy and vent lines, thevenous return cannula is positioned in a vein in contact with arelatively constant stream of blood. Thus, the conventional venous drainmethod is to place the reservoir under the patient and allow blood todrain by gravity. This method is facilitated by the relatively largebore venous return cannulas of 36 French OD or more used in open heartsurgery. A major drawback to the gravity drain, however, is that thesystem must be primed before a return pump can take effect. The onlymeans of enhancing venous return is by increasing the head heightbetween the cannula and the venous reservoir. This is achieved either bylowering the location of the reservoir, limited by the floor, and/or byraising the level of the operating table, both which are limited.

Blood is pumped by a centrifugal or roller pump, for example, from thevenous/cardiotomy reservoir through a blood oxygenator and back to thepatient. The pump assumes the pumping task of the heart and perfuses thepatient's circulatory system. The oxygenator typically directs a flow ofblood across a plurality of permeable fibers which are capable oftransferring oxygen to and carbon dioxide from the blood. The oxygenatoralso usually includes a heat exchange system to regulate theextacorporeal blood temperature. Before reaching the patient, the bloodmay pass through a temperature control monitoring system and along aconduit through an arterial filter and bubble detector, before reachingan arterial cannula positioned in a main artery of the patient.

The perfusion system is typically mounted on a table positioned somedistance from the operating table. Thus, the conduits leading from thepatient to the various components of the perfusion system contain asignificant volume of blood. In addition, the various components such asthe venous cardiotomy reservoir and arterial filter also require acertain volume of blood to function properly. All of these componentsput together require a certain “prime” or volume of blood from thepatient to function. The prime volume can be defined as that volume ofblood outside the patient, or extracorporeal.

The need for a large prime volume is contrary to the best interest ofthe patient who is undergoing the surgery and is in need of a minimumsupply of fully oxygenated blood. Therefore, a significant amount ofresearch and development has been directed toward reducing the primevolume within CPB systems. Some of the areas in which such a reductionof volume can be attained is to reduce the volume of the components,such as the venous cardiotomy reservoir, or blood oxygenator. Anothermeans for reducing the volume of the system is to position the perfusionsetup closer to the patient. One specific example of a CPB system forreducing prime is disclosed in U.S. Pat. No. 5,300,015 to Runge. TheRunge circuit eliminates conventional blood reservoirs and utilizes apulsatile pump which compresses a flexible blood conduit to urge bloodtherethrough. Although prime is reduced, the perfusionist can not view areservoir level to help regulate the proper flow of blood to and fromthe patient.

A new type of perfusion system uses a vacuum in conjunction with gravityto drain blood from the venous system. One such configuration is thesubject of a paper entitled “Trial of Roller Pump-Less CardiopulmonaryBypass System” by Hiroura, et al. of the Department of Thoracic Surgery,Nagoya University School of Medicine, published in conjunction withOwari Prefectural Hospital, both in Aichi, Japan. This referencediscloses a system in which a wall vacuum generates a negative pressureof between −5 and −35 mmHg within a main reservoir, which is connectedto a plurality of individual suction reservoirs and to a venous returnline. Cardiotomy and other suction lines from the patient are attachedto the individual suction reservoirs, and the vacuum pressure withineach one of the suction reservoirs can be regulated independently. Thesystem further includes a centrifugal pump under the main reservoir forpumping blood through the rest of the CPB system and to the patient. Asignificant amount of hardware is needed for this system to regulate andconnect the various pressure chambers.

There exists a need for a reduced prime CPB system which may make use ofexisting hardware and minimizes trauma to the blood.

SUMMARY OF THE INVENTION

In a preferred embodiment, the present invention provides a vacuumassisted venous drainage system, comprising: a reservoir for receivingblood from a venous system of a patient; a source of vacuum; a conduitextending between the source of vacuum and configured to create anegative pressure within the reservoir; a pressure regulator in theconduit; and a vacuum stabilizer positioned in the conduit between thepressure regulator and the reservoir, the vacuum stabilizer allowing airinto the conduit from the exterior thereof to modulate extreme changesin pressure within the conduit, but preventing air from escaping fromthe conduit.

In another embodiment, a vacuum assisted venous drainage system isprovided, comprising: a hard shell venous reservoir for receiving bloodfrom a venous system of a patient; a source of vacuum; a conduitextending between the source of vacuum and configured to create anegative pressure within the reservoir; a pressure regulator in theconduit; and a moisture trap in fluid communication with the conduitbetween the pressure regulator and the hard shell reservoir, themoisture trap serving to collect fluids drawn from the reservoir beforereaching the pressure regulator.

In a further embodiment, the invention discloses a method of surgery,comprising: securing a first cannula percutaneously in a patient;securing a second cannula percutaneously in a patient; connecting thefirst cannula to a venous reservoir blood inlet port; creating anegative pressure in the venous reservoir; regulating the pressurewithin the venous reservoir; and pumping blood from the venous reservoirthrough a blood oxygenator and to the second cannula back to thepatient.

Another aspect of the present invention is a reduced blood/air interfacevenous reservoir, comprising: a rigid container having an inlet adaptedto receive venous blood into an interior space sealed from theatmosphere, the container shaped to contain the blood and form a bloodsurface; an outlet in the rigid container adapted to drain blood to anextracorporeal oxygenation circuit; a vacuum port in the reservoiradapted to be connected to a source of vacuum; and a flexible airimpermeable membrane mounted within the container and defining a closedspace sealed from the interior space of the container, the membranehaving sufficient flexibility so that the closed space expands into theinterior space upon a vacuum being drawn within the container, themembrane configured to expand and contact the blood surface.

Another method provided by the present invention is a method ofcollecting blood within a venous reservoir to reduce the blood/airinterface, comprising: supplying blood to an inlet port of a reservoirhaving a rigid outer container defining an inner space sealed from theatmosphere, the container being shaped to channel the blood and for ablood surface; drawing a vacuum within the rigid outer container;providing a flexible membrane attached within the rigid container anddefining a closed space sealed from the container inner space, so thatthe closed space expands into the interior space upon a vacuum beingdrawn within the container, the membrane configured to expand andcontact the blood surface; and draining the blood from the containerthrough an outlet port.

The present invention is also embodied in a vacuum assisted venousreservoir, comprising: a rigid, sealed outer housing; a flexible, bloodimpermeable reservoir within the housing; an inlet port in thereservoir; a conduit attached to the inlet port and in communicationwith the interior of the reservoir, the conduit passing through a sealedopening in the housing and being connected to a source of venous blood;a vacuum conduit extending between a source of vacuum and the interiorof the housing through a sealed opening; and a pressure regulatorbetween the vacuum conduit and vacuum source.

Further objects and advantages of the present invention shall becomeapparent to those skilled in the art upon reading and understanding thefollowing detailed description of a presently preferred embodiment ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a cardiopulmonary bypass system ofthe prior art;

FIG. 2 is a schematic representation of a minimally invasivecardiopulmonary bypass system of the present invention utilizing avacuum-assisted hard-shell venous reservoir;

FIG. 3 is an enlarged perspective view of one type of hard-shelledreservoir which may be adapted for use with the present invention;

FIG. 4a is a cross-sectional view of a hard-shelled venous reservoiradapted for vacuum-assisted venous drainage and having reduce blood/airinterface, prior to a vacuum being applied;

FIG. 4b is a cross-sectional view of the reservoir of FIG. 4a after avacuum is applied;

FIG. 5a is a cross-sectional view of another embodiment of ahard-shelled venous reservoir adapted for vacuum-assisted venousdrainage and having reduce blood/air interface, prior to a vacuum beingapplied;

FIG. 5b is a cross-sectional view of the reservoir of FIG. 5a after avacuum is applied;

FIG. 6 is a schematic representation of a further embodiment of aminimally invasive cardiopulmonary bypass system of the presentinvention utilizing a vacuum-assisted soft-shell venous reservoir;

FIG. 7 is a schematic representation of a combination pressure-reliefvalve and vacuum stabilizing device for use with vacuum-assistedreservoirs of the present invention; and

FIG. 8 is a schematic representation of a further embodiment of aminimally invasive cardiopulmonary bypass system of the presentinvention utilizing a vacuum-assisted soft-shell venous reservoir.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides vacuum-assisted venous drainage intoreservoirs of various types. As shown and described herein, both hard-and soft-shelled reservoirs may be used, although those of skill in theart will recognize that various other types of reservoirs may also beadapted for vacuum-assisted drainage. Furthermore, the reservoirs asdepicted in the present invention are combined cardiotomy and venousreservoirs, but venous reservoirs separate from cardiotomy reservoirsmay also be adapted for vacuum-assisted drainage. Finally, variouscomponents of conventional cardiopulmonary bypass systems may be usedwith the vacuum-assisted venous drainage reservoirs of the presentinvention, so that those illustrated are not to be considered limiting,and any other conventional components typically used in CPB systems andomitted from the description and drawings may be included.

With reference to FIG. 2, a vacuum-assisted venous drainage system 20 isillustrated connected to a patient 22 and shown as part of a largercardiopulmonary bypass system 24. The patient 22 is placed on thecardiopulmonary bypass system 24 while undergoing a number ofprocedures, a minimally invasive heart surgery procedure is shown. Inminimally invasive surgery, the chest cavity is not opened, but insteadthe various instruments and fluid communication conduits are insertedinto the chest cavity through one or more small openings therein.Minimally invasive surgery greatly reduces the recovery time for suchheart surgeries, and is rapidly gaining popularity in the medicalcommunity, and in the population at large. Although the presentinvention is illustrated in conjunction with minimally invasive surgery,similar advantages of reduced prime volume and less blood trauma arerealized when using the invention in conjunction with more conventionalopen heart surgery techniques.

Hard Shell Reservoir Vacuum-Assisted Venous Drainage System

FIG. 2 illustrates a venous return cannula 26 extending into thepatient's chest cavity and being connected on a proximal end to a venousreturn line 28. The venous return cannula 26 may be of a variety ofconfigurations and sizes, but for minimally invasive surgery ispreferably selected from a group of cannulas previously used forpediatric or small adult patients. For example, a typical large borecannula for open heart surgery may have an OD 36 French on its tip,while cannulas for pediatric or small adult applications have tips ofbetween 18 and 26 French. Such small bore cannulas are not presentlyused for conventional gravity impelled venous drainage in heart surgeryon large adults because of their reduced volumetric flow capacity. Withthe present vacuum assisted drainage system 20, the cannulas 26 can beever smaller in size, which facilitates the minimally invasive surgerytechniques. In conjunction with the vacuum drainage system 20, modernthin-walled cannulas are preferably used which are fabricated by anextrusion manufacturing process, as opposed to a dipping process.Extruding the venous return cannula enables a thinner walledconstruction, and associated larger lumen size for any particular outerdiameter. Such cannulas can be obtained from Research Medical Inc., ofSalt Lake City, Utah, a subsidiary of Baxter International Inc. ofDeerfield, Ill.

The vacuum assisted drainage system 20 comprises the aforementionedcannula 26 and return line 28, in combination with a venous reservoir 30supplied with a negative pressure from a wall vacuum 32. A wall vacuum32 is preferred, as almost all operating rooms already have such vacuumsystems in place. Alternatively, however, those of skill in the art willunderstand that other sources of vacuum may be used. A vacuum regulator34 and indicator 36 are provided in a vacuum line 38 between the wallvacuum 32 and the reservoir 30. The regulator 34 may be controlled by acontroller 40 which receives input from sensors in various locationswithin the system 20. For example, a pressure sensor 42 may be providedin the venous return line 28 to sense overpressure caused by blockage inthe line or cannula 26, or other such occlusion. Another sensor 44 maybe provided to sense the pressure within the reservoir 30. In onespecific example, the wall source 32 supplies negative pressure ofapproximately 170 mmHg, and the regulator 34 steps the negative pressuredown to between 0 and 75 mmHg within the reservoir 30.

Between the vacuum regulator 34 and the reservoir 30, a combinationpressure relief valve and vacuum stabilizer unit 50 is provided. Theunit 50 may take a variety of forms, but provides the function ofpressure relief if pressure within the reservoir 30 reaches a thresholdvalue. In addition, the unit 50 provides the function of stabilizing thevacuum supplied to the reservoir 30 by resisting large changes in themagnitude of the vacuum. One particular embodiment of the combinedpressure relief valve in vacuum stabilizer unit 50 is illustrated anddescribed with respect to FIG. 7. Of course, the pressure relief valveand vacuum stabilizer may be provided separately and not as a unit.

Farther down the vacuum line 38 toward the reservoir 30, a moisture trap52 is provided. The moisture trap 52 receives the vacuum line 38 at anozzle in an upper end, which nozzle is in communication with a tube 54a extending downward into a chamber. A second tube 54 b is in fluidcommunication with a second nozzle and shunt line 56. The shunt linecontinues from the moisture trap 52 to a vacuum port 58 provided in thereservoir 30. The moisture trap 52 collects any liquids or other fluidvapor escaping from the reservoir 30 so as not to contaminate thecomponents in the vacuum line 38, or the wall vacuum 32. A number ofmoisture traps 52 are available, and the presently illustrated moisturetrap attached rigidly to the reservoir 30 is a preferred form only.

One particularly suitable reservoir 30 for use in the present vacuumassisted drainage system 20 is illustrated in FIG. 3. The reservoir 30includes a hard shelled, generally cylindrical canister 70 which has agradual outward taper in an upward direction. The reservoir shown isModel HSR 4000 manufactured by Bentley, Inc., of Irvine, Calif., asubsidiary of Baxter International Inc., of Deerfield, Ill. Other hardshelled reservoirs, such as the Bentley BMR-4500, may be suitable. Thereservoir 30 includes a cap 72 having a plurality of ports communicatingwith the interior of the canister 70. More specifically, the reservoir30 includes the vacuum port 58 and venous return port 74. A plurality ofcardiotomy ports 76 on an upper housing may be attached to cardiotomylines extending to the patient 22, although these lines are not shownfor clarity. In a preferred embodiment, the vacuum assisted hard shelledsystem 20 is incorporated into a reservoir 30 such as the HSR 4000,which was previously used for gravity drainage. To facilitate gravitydrainage, the vacuum port 58 was left open as a vent port to allow forthe escape of air within the canister 70 as blood drains into thereservoir. In the present invention, a regulated vacuum source isconnected to the vacuum port 58 so that the canister 70 is closed andsealed to the atmosphere, and the remaining components of thevacuum-assisted system 20 are implemented.

The preferred reservoir 30 is a combined cardiotomy and venous returnreservoir, with concentric inner cardiotomy chamber, intermediate venouschamber, and outer collection chamber. These chambers can be seen inFIGS. 4 and 5, and are conventional in the art. In use, cardiotomy fluidalong with any bone or particulate matter enters the inner cardiotomychamber and is filtered before entering the intermediate venous chamber.With reference to FIG. 3, both the combined cardiotomy and venous bloodis then passed through a defoamer sock 80 and into the outer collectionchamber before exiting the reservoir through a lower exit port 78. Thedefoamer sock 80 comprises a generally cylindrical exterior shape whichdefines a space 82 with the canister wall 70.

With reference again to FIG. 2, the CPB circuit 24 also comprises areservoir drain line 82 extending to a pump 84 controlled by acontroller 86. The pump 84 may be of a variety of types, includingroller pumps or centrifugal pumps. Blood is then pumped through a shuntline 88 to an oxygenator 90. Again, the oxygenator may take a variety offorms, but is preferably a SpiralGold oxygenator manufactured byBentley, Inc. The oxygenated blood continues past an arterial filter 92and to an arterial return line 94 terminating in an arterial cannula 96.Other components, such as bubble detectors, may be provided in thereturn line 94, as is well known in the art.

Vacuum-Assisted Venous Drainage Performance

The venous reservoir 30 is characterized by the hard outer shell 70which is sufficient to withstand any pressures generated within from thewall vacuum 32 and regulator 34. The negative pressure developed withinthe hard shelled reservoir 30 creates a negative pressure within thevenous return line 28, which in turn pulls blood from the vein in whichthe cannula 26 is placed. Because of the vacuum assisted venous returnsuction, the reservoir 30 may be positioned at various locations withrespect to the patient 22. More specifically, as opposed to priorgravity drain reservoirs, the reservoir 30 may even be positioned abovethe patient 22, although a small elevation below the patient ispreferred. Because the reservoir 30 need not be close to the ground, asbefore, it can be hung adjacent or behind the patient in variouslocations previously not possible. This greatly increases theflexibility of the operating room set up, and significantly reducesprime volume by reducing the length of tube from the cannula 26 to thereservoir 30.

Volumetric flow results for different cannula sizes, reservoir headheights below the patient, and vacuum magnitudes are presented below inseveral tables. These results were obtained using a Bentley HSR 4000reservoir and a container of bovine blood. A roller pump is used to pumpthe blood from the reservoir back to the container of blood. The bloodused had a hematocrit (HCT) level of about 32.5. The head height is thelevel of the reservoir below the container. The vacuum was connected tothe conventional vent port, and all other input ports were plugged. Thereservoir level was maintained at 2000 ml and measurements taken atsteady state conditions. The range of blood flow through operatingbypass systems varies, and a rough estimate for normal adults having ahematocrit level of 25-40 is between 3.5 and 4.5 lpm. It is apparentfrom these data, therefore, that flow rates sufficient for adult bypasssurgery can be obtained with the vacuum-assisted reservoir system of thepresent invention using cannulas of reduced size. This developmentpromises to revolutionize bypass procedures toward minimally invasivetechniques.

TABLE I (Cannula Size = 24 Fr) Pressure Measurements Head Height VacuumPump Flow @ Cannula End @ Reservoir (inch) (mmHg) (1 pm) (mmHg) End(mmHg) 6 0 1.40 −21 6 6 −15 2.15 −35 −6 6 −30 2.61 −49 −20 6 −46 3.25−62 −33 6 −61 3.63 −75 −46 6 −75 4.06 −88 −59 12 0 1.75 −22 15 12 −152.39 −44 −5 12 −30 2.80 −59 −20 12 −45 3.50 −72 −32 12 −61 3.98 −88 −4712 −76 4.36 −98 −58 18 0 2.53 −47 3 18 −15 2.96 −55 −5 18 −30 3.35 −69−18 18 −45 3.75 −82 −31 18 −60 4.25 −94 −44 18 −74 4.67 −105 −55

TABLE II (Cannula Size = 22 Fr) Pressure Measurements Head Height VacuumPump Flow @ Cannula End @ Reservoir (inch) (mmHg) (1 pm) (mmHg) End(mmHg) 6 0 1.10 −18 6 6 −15 1.53 −32 −7 6 −30 1.91 −46 −20 6 −45 2.31−59 −32 6 −60 2.63 −72 −45 6 −75 2.98 −85 −57 12 0 1.62 −34 3 12 −151.87 −44 −6 12 −30 2.28 −58 −19 12 −45 2.64 −72 −33 12 −60 2.97 −83 −4512 −75 3.26 −98 −58 18 0 1.75 −41 7 18 −15 2.18 −55 −6 18 −30 2.56 −70−19 18 −45 2.82 −82 −32 18 −60 3.15 −95 −44 18 −75 3.52 −109 −57

TABLE III (Cannula Size = 20 Fr) Pressure Measurements Head HeightVacuum Pump Flow @ Cannula End @ Reservoir (inch) (mmHg) (1 pm) (mmHg)End (mmHg) 6 0 .83 −32 6 6 −15 1.19 −46 −11 6 −30 1.50 −59 −26 6 −451.75 −73 −45 6 −60 2.06 −86 −61 6 −75 2.30 −99 −77 12 0 1.20 −47 −1 12−15 1.48 −57 −14 12 −30 1.79 −71 −30 12 −45 2.03 −84 −46 12 −60 2.31 −98−63 12 −75 2.53 −111 −78 18 0 1.75 −41 7 18 −15 2.18 −55 −6 18 −30 2.56−70 −19 18 −45 2.82 −82 −32 18 −60 3.15 −95 −44 18 −75 3.52 −109 −57

TABLE IV (Cannula Size = 18 Fr) Pressure Measurements Head Height VacuumPump Flow @ Cannula End @ Reservoir (inch) (mmHg) (1 pm) (mmHg) End(mmHg) 6 0 .67 −34 3 6 −75 1.81 −104 −80 12 0 .89 −45 3 12 −15 1.13 −57−11 12 −30 1.43 −72 −29 12 −45 1.67 −86 −45 12 −60 1.94 −102 −65 12 −752.12 −115 −80 18 0 1.00 −55 3 18 −15 1.22 −70 −16 18 −30 1.62 −84 −34 18−45 1.79 −96 −47 18 −60 2.00 −112 −67 18 −75 2.17 −127 −86

Reduced Blood/Air Interface

Hard shelled reservoirs are generally used when larger volumes, reducedresistance to venous return. and accurate blood volumes within thereservoir are needed. These units are typically larger than so-called“soft shell” reservoirs, and since the sides of the reservoir are rigid,flow is less restricted and volume levels at a certain liquid height canbe measured and labeled on the reservoir. Hard shelled reservoirs,however, operate partially full so that a blood/air interface at the topof the liquid level exists. In contrast, soft shelled reservoirs areflexible and allow air to be excluded which substantially reduces theblood/air interface. Furthermore, the flexible nature of soft shellreservoirs restrict blood flow and the ability to directly measure thevolume of blood within. The present invention provides for a directvisual volumetric measurement of blood while also reducing the blood/airinteraction. Reducing blood/air interaction reduces the activation ofblood and provides for less blood related complications during bypasssurgery.

With reference to FIGS. 4a and 4 b, the present invention provides ahard shelled reservoir 100 which includes a mechanism for reducing theblood/air interaction, while also providing the benefits of a directvisual volume measurement. The reservoir 100 may be a modified HSR 4000reservoir manufactured by Bentley, Inc., or other suitable hard shelledreservoir. In this respect, and as mentioned previously, the reservoirincludes an outer canister 102, an inner venous canister 104, and aninnermost cardiotomy canister 106. FIG. 4a on the left illustrates thereservoir 100 prior to any blood flow therethrough, while FIG. 4b showsblood entering through a venous return line 108, and fluid entering thecardiotomy canister 106 through a suction line 110. The reservoir 100 issupplied with a negative pressure, as described above with respect toFIGS. 2 and 3, although the vacuum port is not shown in these drawings.

To reduce the blood/air interface, a highly flexible, air impermeablemembrane 112 is mounted in an upper area of the region between the outercanister 102 and the venous canister 104. In the illustrated embodiment,the membrane 112 comprises an annular tube of highly flexible materialwith a generatrix or circumferential edge being attached to theunderside of the reservoir cap 114. The tubular membrane 112 may beattached by an adhesive, solvent bonding or other expedient methods. Thetubular membrane 112 is constructed from a material such as a lowdurometer polyurethane or silicone, and is sufficiently flexible toexpand and fill the inside of the reservoir 100 between the outercanister 102 and the venous canister 104 above a blood surface 116, asseen in FIG. 4b. Under atmospheric pressure, the tubular membrane shouldcontain a small amount of air or inert gas within. The vacuum createdwithin the reservoir 100 expands the tubular membrane 112 to fill thespace above the blood surface 116. As the vacuum level in the reservoir100 is varied leading to changes in blood level, the membrane 112 willexpand or contract and maintain contact with the surface of the bloodwithout significantly hindering flow. Preferably, the membrane 112 isconfigured to contact substantially all of the surface of the blood inthe annular space outside of the venous canister 104. This essentiallylimits blood/air contact and associated blood related complications tothe spaces within the venous canister 104.

FIGS. 5a and 5 b illustrate another hard shelled reservoir 100′ which isconfigured identically to the reservoir 100 described in FIG. 4. Commoncomponents of the reservoir 100′ are thus indicated by a prime. Aflexible air impermeable membrane 112′ is substituted for the tubularmembrane 112 shown in FIG. 4a. Instead of being a contained tube, themembrane 112′ is formed of a sheet of highly flexible material with itslongitudinal edges attached to the underside of the cap 114′. In allother respects, the flexible membrane 112′ acts in the same manner asthe earlier described tubular membrane, and expands as in FIG. 5b tosubstantially fill the space between the outer canister 102′ and thevenous canister 104′. Again, this substantially reduces the blood/airinterface within the reservoir 100′.

FIGS. 4a and 4 b also illustrate a system for accurately determining thevolume of blood within the reservoir. The system comprises an ultrasonicsensor 120 mounted to a lower wall 122 of the canister 102. The sensor120 is mounted to be directly underneath the space between the outercanister 102 and the venous canister 104. The sensor 120 providesinformation to a control system 124 which may be in communication withcontrol systems for the vacuum generator, or blood pump, such as thecontrols 40 and 86 illustrated in FIG. 2. The sensor detects the levelof blood within the reservoir 100, such as the blood surface 116 in FIG.4b, and sends that information to a processor which is able to computethe volume of blood in the reservoir. Such a sensor is shown anddescribed in U.S. Pat. Nos. 5,303,585 and 5,586,085, both to Lichte.Accurate knowledge of the blood level within the reservoir 100 enablesrapid response to varying blood flow conditions so that the vacuum maybe adjusted to increase or decrease the flow, or for other purposes suchas metering an anticoagulant added to the extracorporeal blood to reduceclotting.

Soft Shell Reservoir Vacuum-Assisted Venous Drainage System

With reference to FIG. 6, a CPB procedure 130 is shown utilizing a softshell reservoir venous drain system 132. The system 132 includes a softshell reservoir 134 located completely within a rigid housing 136. Avenous cannula 138 placed in a vein of the patient is in fluidcommunication with the reservoir 134 via a venous return line 140. Thereturn line 140 enters the housing 136 through a sealed aperture 142. Acardiotomy line 144 leading from a cardiotomy filter (not shown) may bejoined to the venous return line at a Y-junction 146. A drain line 148connects the output of the reservoir 134 with a pump 150, which may havea controller 152. The pump 150 sends blood through an oxygenator 154, anarterial filter 156, an arterial line 158, and finally to an arterialcannula 160 for perfusing the patient's arterial system.

The blood from the venous system is pulled into the reservoir 134 by anegative pressure gradient in the venous return line 140 created by anegative pressure in the reservoir. A source of vacuum 162, such as awall vacuum, is connected to the interior of the housing 136 via apressure regulator 164 and vacuum line 166. The vacuum line 166 projectsthrough a sealed fitting 168 into the housing 136. A negative pressurecreated in the housing 136 tends to inflate the reservoir 134 which inturn, creates a vacuum therein to draw the venous blood from thepatient. The reservoir 134 may be of a variety of types, but ispreferably one of the following made by Bentley, Inc: BMR-800 Gold orBMR-1900 Gold.

A second pressure regulator 170 communicates with the interior of thereservoir 134 via a conduit 172 entering the housing 136 through asealed fitting 174. The pressure within the conduit 172 is regulated tomaintain a pressure differential between the interior and exterior ofthe reservoir. This secondary pressure regulation may be used to adjustvenous return flow rates.

A third pressure regulator 180 communicates with the interior of thereservoir 134 via a conduit 182 entering the housing 136 through asealed fitting 184. The pressure within the conduit 182 is regulated togently pull a suction at the very top of the reservoir 134 to remove anyair which may be trapped within. Microbubbles in the blood sometimescombine to form significant air pockets which must be removed before theblood is sent back to the patient. Further, the blood/air interface isdetrimental and is preferably eliminated.

As an alternative to the air removal conduit 182, a membrane 190 ofsandwiched layers of hydrophobic and hydrophilic materials may be formedinto the upper wall of the reservoir 134. This membrane 190 wouldfacilitate passive venting of air from the reservoir 134 due to thevacuum generated within the housing 136.

As with the earlier described hard shell reservoir embodiment, varioussensors may be positioned around the system 130 for monitoringpressures, flows, temperatures, blood levels, etc. A microprocessor 194may be provided to control the three pressure regulators 164, 170, and180 and enhance the efficiency of the system. The microprocessor 194 mayalso be connected to the pump controller 152, oxygenator 154, or otherdevice such as a heat exchanger (not shown).

Pressure Relief Valve/Vacuum Stabilizer

Each of the conduits 166, 172 and 182 between the vacuum regulators andhousing 136 include a pressure relief and vacuum stabilizer unit 200, apreferred form of which is illustrated in FIG. 7. The unit 200 comprisesa chamber 202 open at opposite ends to nipples 204 a and 204 b. Thenipples 204 a,b are used to connect the units 200 in series in one ofthe conduits 166, 172 and 182. The unit 200 further includes a pressurerelief valve 206 and a vacuum stabilizing valve 208. The pressure reliefvalve 206 cracks open at a very low pressure differential threshold andbleeds air out of the line, in response to buildup of positive pressure.The vacuum stabilizing valve 208 continuously bleeds air into the systemat a flow rate proportional to the level of vacuum to be maintained bythe particular pressure regulator 164, 170 or 180. When the level ofvacuum increases within the system to a predetermined threshold value,the vacuum stabilizing valve 208 yields (the opening increases)proportionately to allow an increased amount of air to bleed into thesystem, thereby compensating for the increased level of vacuum. Thishelps reduce large swings in vacuum, thus the stabilizing effect. In apreferred form, both the pressure relief valve 206 and a vacuumstabilizing valve 208 are conventional duckbill valves chosen for thedesired pressure threshold. This greatly reduces the cost of the system.

Key Hole Surgery

One type of minimally invasive surgery gaining in acceptance is aso-called “key hole” technique 220 shown in FIG. 8. A vacuum-assistedsoft shell reservoir system 132, as shown and described with referenceto FIG. 6, is connected to a patient with the venous return line 140 andarterial perfusion line 158 joining at 222. The two lines are introducedinto the patients femoral vein using a dual-stage cannula 224. Thearterial perfusion line 158 extends up through the patient's vasculatureto a location suitable for oxygenated blood perfusion. With this type ofsurgery, only one access incision is needed, and the perfusion lines arepositioned out of the way of the patient's thorax, where otherinstruments or probes may be inserted to operate on the heart.

Flow Control

The pressure differential created by the application of a negativepressure at the reservoir end of the venous return line can be regulatedto enable control over venous return, independent of the relativepositioning of the reservoir to the operating table. As mentioned above,various sensors may be positioned at critical locations for measuringpressure, temperature, flow rates, blood levels, etc. The sensors may beconnected to a control system with output to a number of actuators suchas the vacuum regulators, circulation pump, secondary regulators, heatexchangers, etc. Certain variables can be sensed and/or controlled withappropriate feedback loops, preferably coded into a programmablemicroprocessor. These variables include the amount of vacuum applied tothe reservoir (or housing around the reservoir), the pump speed or pulserate, the threshold level of vacuum allowed in the venous return line,and the reservoir minimum and maximum volumes. Such a sophisticatedcontrol system would result in reduced hemodilution (prime volume) byenabling reduced cannula sizes, reduced tubing sizes and lengths,accurate control of venous drainage, and enhanced control of arterialreturn. In addition, benefits gained to perfusion control includepositive control of venous drainage and arterial perfusion, andmicroprocessor control of flow rates. In addition, the microprocessorcould be adapted to measure and control circuit temperature andoxygenation. With adequate fail-safes built into the system, thetraditional role of the perfusionist is greatly reduced.

Pediatric Applications

Although cannula sizes for older children and teens range from 18-26 Fr,smaller cannulas are often used for infants and newborns. For example,Research Medical, Inc. (RMI), of Salt Lake City, Utah, provides a lineof cannulas down to 8 Fr in size. French (Fr) is a term for the outsidediameter (OD) of the cannula, and the conversion to metric is: 1 mm=πFr.Thus, an 8 Fr cannula has an OD of 2.54 mm. The bore size of aparticular Fr cannula will depend on the cannula wall thickness. Asmentioned above, RMI has developed an extrusion process which brings thewall thickness of an 18 Fr cannula down to 0.018 inch (0.457 mm) frombetween 0.022-0.027 inch (0.559-0.686 mm) for earlier designs fabricatedby conventional dipping methods. An 18 Fr cannula has an OD of 5.73 mm.With a wall thickness of 0.457 mm, the ID is 4.816 mm. Conventional 18Fr cannulas would have a maximum ID of 4.612 mm. The increase in thecross-sectional flow area through extruded cannulas is thus 9%. Thisincrease, in combination with drawing a negative pressure in thecannula, greatly facilitates the use of smaller and smaller cannulas. Itshould be emphasized that cannulas smaller than the currently available8 Fr size may become viable for neonatal care, for example, with thevacuum-assisted drainage and thin walled cannulas. In other words, thebenefits of the present invention will be realized by patients of allsizes. Moreover, the reduction in extracorporeal blood prime volumewhich is realized by locating the reservoir closer to the vein is mostsignificant for neonatals and infants, who have a relatively much lessamount of blood in their vasculatures. Neonatals, for example, may onlyrequire a blood flow through the CPB circuit of less than 1 lpm.

Benefits to Vacuum-Assisted Venous Drainage

The present invention is expected to achieve the following benefits forconventional cardiopulmonary bypass surgery:

1. Venous return flow rates will no longer depend on the physicallocation of the reservoir with respect to the operating table, providingopportunities for miniaturization of the entire CPB circuit.

2. A miniaturization of the CPB circuit would lead to minimizing bloodcontact with foreign surfaces thereby reducing complications associatedwith immune response such as platelet depletion, complement activationand leukocyte activation.

3. Since venous flow is initiated by the application of negativepressure, the venous return line need not be primed. This leads to asubstantial reduction in priming volume of the CPB circuit, resulting inreduced hemo-dilution of the patient. This aids in the recovery of thepatients physiological status after surgery.

4. Surgeon acceptance of the use of vacuum for venous return, could leadto suction systems taking the place of roller pumps in otherapplications. This potentially could lead to elimination of roller pumpsthat support the sucker, sump and vent functions that fall under theresponsibility of the perfusionist. This would result in minimizingblood trauma caused by roller pumps. Additionally, this would alsofree-up valuable floor space in the vicinity of the operating table.

In addition, the present invention is expected to achieve the followingbenefits for Minimally Invasive Surgery:

1. Vacuum assist will help achieve higher flow rates through smallercannulae. This will allow the use of smaller cannulae thereby providingthe surgeon greater access to the site of the surgery. This could alsopotentially eliminate the need for larger size cannulae.

2. MICS techniques, such as the Key Hole concept, use the femoral veinas access port for CPB circuitry. This technique imposes stringentlimitations on the cross-section and length of the catheter/cannula usedfor venous return. Vacuum assist can replace centrifugal pumps toenhance venous drainage in these applications.

It is understood that the examples and embodiments described herein andshown in the drawings represent only the presently preferred embodimentsof the invention, and are not intended to exhaustively describe indetail all possible embodiments in which the invention may take physicalform. Indeed, various modifications and additions may be made to suchembodiments without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A vacuum assisted venous reservoir, comprising: arigid, sealed housing, said housing being sealed such that a relativevacuum can be maintained between the interior and the exterior of thehousing; a flexible, blood impermeable reservoir positioned within theinterior of the housing; an inlet port in communication with theinterior of the reservoir; a venous blood conduit attached to the inletport and in communication with the interior of the reservoir, the venousblood conduit passing through a first sealed opening in the housing andbeing connected to a source of venous blood such that venous blood canpass from the source of venous blood to the interior of the reservoir; avacuum conduit extending through a second sealed opening in the housing,the vacuum conduit being in communication with a source of vacuum andthe interior of the housing such that vacuum pressure can be applied tothe interior of the housing; a pressure regulator in communication withthe vacuum conduit and vacuum source for regulating the vacuum pressurein the interior of the housing; a second vacuum conduit extendingthrough a third sealed opening in the housing and in communication witha source of vacuum and the interior of the reservoir such that vacuumpressure can be applied to the interior of the reservoir; and a secondpressure regulator in communication with the second conduit and thesecond vacuum source for regulating the vacuum pressure in the interiorof the reservoir.
 2. The reservoir of claim 1, further including amembrane formed into a wall of the reservoir, said membrane beingconfigured to vent air from the interior of the reservoir.
 3. Thereservoir of claim 2, whereby said membrane is made of sandwiched layersof hydrophobic and hydrophilic materials.
 4. The reservoir of claim 1,wherein the flexible reservoir further includes an air permeablemembrane for venting air from within the reservoir to the interior ofthe housing.
 5. The reservoir of claim 1, wherein the second vacuumconduit communicates with the uppoermost portion of the interior of thereservoir such that vacuum pressure applied to the interior via thesecond vacuum conduit can remove air trapped within the reservoir.